a tenth part (originally of produce) due as support of the clergy, c. 1200, from Old English teogoþa (Anglian), teoþa (West Saxon) "tenth," from Proto-Germanic *tegunthan, from PIE *dekmto-, from PIE root *dekm- "ten." Retained in ecclesiastical sense while the form was replaced in ordinal use by tenth.
Old English teoþian "to pay one-tenth," from the root of tithe (n.). As "to impose a payment of a tenth," late 14c. Related: Tithed; tithing.